AUSTIN, Texas—A pilot slammed his small plane into a seven-story building that housed the local office of the Internal Revenue Service Thursday, apparently killing himself and one agency employee, in what federal officials described as a deliberate suicide attack amid a long-running tax dispute.

Investigators are looking into whether the pilot, 53-year-old Andrew Joseph Stack, also set his house on fire before taking off in his single-engine Piper Dakota around 9:40 a.m. local time.

Officials said they were evaluating an antigovernment manifesto posted on the Internet earlier Thursday, signed "Joe Stack," which suggested he planned the crash. "Violence not only is the answer, it is the only answer," the author wrote toward the end of a tirade against the IRS posted at 9:12 a.m. on a Web site registered to Mr. Stack. He described himself as a contract software engineer.

By late Thursday night, emergency crews had found two bodies in the wreckage, the Associated Press reported. Authorities said earlier that the pilot who crashed into the building was presumed dead and that one worker in the building was missing. Thirteen people were injured, including two who remained hospitalized for critical injuries. More than 100 IRS employees work at the building.

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Officials labeled the crash a criminal, not terrorist, attack. "I consider this a criminal act by a lone individual," said Police Chief Art Acevedo.

Still, the North American Aerospace Defense Command launched two F-16 fighter aircraft to patrol the air after the crash. Spokesman Jamie Graybeal called it "a prudent precaution and consistent with our response to recent similar air incidents."

The White House said President Barack Obama was briefed on the plane crash after noon.

The crash came less than an hour after an explosion ripped through Mr. Stack's two-story red brick house in a subdivision a few miles from the site of the crash, neighbors said. The house was quickly engulfed in flames and was gutted by the fire.

Mr. Stack's family declined to comment, according to police officers stationed at a house across the street from the burned-out home. According to government records, Mr. Stack bought a house in Austin in 2007 and was married in July of that year to Sheryl Mann.

In the 3,200-word manifesto, posted on the Web site Embeddedart.com, the author, who identified himself as "Joe Stack," described himself as a recently married, struggling, contract software engineer who left Los Angeles, where he couldn't find work, but didn't have much better luck in Austin.

The author complained bitterly about the IRS, blaming the agency for eroding his retirement savings and causing him financial troubles for years.

First images from Austin, TX, where a small plane has crashed into a building. Courtesy Fox News.

IRS employee Ray Cano listened to the news on his cellphone outside after a plane crashed into his office building. Eli Meir Kaplan for The Wall Street Journal

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Because of his expenses and lack of income, Mr. Stack wrote, he didn't file a tax return, prompting an IRS audit that cost him $10,000. He wrote that he had other problems involving "Sheryl's unreported income."

"I would only hope that by striking a nerve that stimulates the inevitable double standard, knee-jerk government reaction that results in more stupid draconian restrictions people wake up and begin to see the pompous political thugs and their mindless minions for what they are," the author wrote.

"Well, Mr. Big Brother IRS man, let's try something different; take my pound of flesh and sleep well," he wrote in conclusion, signing the posting, "Joe Stack (1956-2010)." The Web site was taken down on Thursday afternoon after a request from the Federal Bureau of Investigation, according to the president of the Web-hosting service.

The plane crash sent a plume of dark smoke into the air and caused highway shutdowns about 10 miles north of the state capitol. Witnesses said they saw the plane rapidly approaching the office building, one of four seven-story buildings with black windows, in an office park which sits next to a six-lane corridor near a high-end shopping center.

Matt Downs, a software salesman with offices about a quarter of a mile away, said the plane banked at high speed with the engine at full throttle. It hit between the second and third floors.

Alan Fletcher, who was working in an adjacent building, ran into the building and encountered shattered glass everywhere and dazed employees emerging from the smoke.

"The whole building shook; it felt like a car hit our building," said Camille Ziegelhofer, who works at a software company in a nearby building.

Richard Lee, an IRS manager, said the explosion blew out the windows and knocked him to the floor, bruising his back. He helped others in the building get out, he said, but couldn't get to part of his floor because of neck-high flames.

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